Maize rough dwarf virus (MRDV) is a plant reovirus with a complex structure: the genome, comprising 10 pieces of ds-RNA, is contained in 2 concentric protein capsids, both having icosahedral symmetry (1). Ultrastructural information has been obtained mostly by negative staining of virus particles in crude or purified extracts or, to a lesser extent, by examining ultrathin resin sections of infected cells (Fig. 1). No information however was ti11 now available on the fine structure of MRDV inner core which appeared, both in negative staining and resin section, as a more or less electron-opaque sphere with a diameter of about 40 nm. To improve our knowledge of this structure we examined MRDV particles in ultrathin cryosections of maize tissue after staining with neutra1 uranyl acetate (UA) and coating with methyl cellulose (2). With this technique the virue core was resolved in a series of repeating geometrical profiles, some of which were easily identified as having 2-, 3- or 5-fold symmetry (Fig. 2). From these images it was possible to obtain a 3-D reconstruction of the virus core, which appeared to be a dodecahedron with 12 pentagonal faces. The strong electron-opacity given to this structure by UA confirms that it is mainly made of nucleic acid, for which UA is known to have a selective affinity, To obtain further evidence, cryosections were incubated in pancreatic RNase in the presence of high or low salt concentration, respectively 2x or 0.01xSSC (saline sodium citrate), remembering that ds-RNA is susceptible to RNase in media of low but not high salt concentration (3). After treatment with RNase in 0.01xSSC the geometrical structures inside virus particles totally disappeared, leaving white holes (Fig. 3). After RNase in 2xSSC some of them remained, others were only partially digested (Fig. 4 ) , confirming that these structures represent the virus genome, made of ds-RNA. The type of symmetry resulting for the virus core is not in contrast with that suggested for the inner capsid, provided that each face of the dodecahedron is in correspondence with each vertex of the icosahedron.
CYTOCHEMICAL STUDIES ON A PLANT REOVIRUS IN CRYOSECTIONS OF ITS HOST PLANT.
CACIAGLI;
1991
Abstract
Maize rough dwarf virus (MRDV) is a plant reovirus with a complex structure: the genome, comprising 10 pieces of ds-RNA, is contained in 2 concentric protein capsids, both having icosahedral symmetry (1). Ultrastructural information has been obtained mostly by negative staining of virus particles in crude or purified extracts or, to a lesser extent, by examining ultrathin resin sections of infected cells (Fig. 1). No information however was ti11 now available on the fine structure of MRDV inner core which appeared, both in negative staining and resin section, as a more or less electron-opaque sphere with a diameter of about 40 nm. To improve our knowledge of this structure we examined MRDV particles in ultrathin cryosections of maize tissue after staining with neutra1 uranyl acetate (UA) and coating with methyl cellulose (2). With this technique the virue core was resolved in a series of repeating geometrical profiles, some of which were easily identified as having 2-, 3- or 5-fold symmetry (Fig. 2). From these images it was possible to obtain a 3-D reconstruction of the virus core, which appeared to be a dodecahedron with 12 pentagonal faces. The strong electron-opacity given to this structure by UA confirms that it is mainly made of nucleic acid, for which UA is known to have a selective affinity, To obtain further evidence, cryosections were incubated in pancreatic RNase in the presence of high or low salt concentration, respectively 2x or 0.01xSSC (saline sodium citrate), remembering that ds-RNA is susceptible to RNase in media of low but not high salt concentration (3). After treatment with RNase in 0.01xSSC the geometrical structures inside virus particles totally disappeared, leaving white holes (Fig. 3). After RNase in 2xSSC some of them remained, others were only partially digested (Fig. 4 ) , confirming that these structures represent the virus genome, made of ds-RNA. The type of symmetry resulting for the virus core is not in contrast with that suggested for the inner capsid, provided that each face of the dodecahedron is in correspondence with each vertex of the icosahedron.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
